Removal of Ammonia Emissions via Reversible Structural Transformation in M(BDC) (M = Cu, Zn, Cd) Metal–Organic Frameworks

NH₃ is the most important gaseous alkaline pollutant, which when accumulated at high concentrations can have a serious impact on animal and human health. More importantly, NH₃ emissions will react with acidic pollutant gases to form particulate matter (PM_2.5) in the atmosphere, which also poses a huge threat to human activities. The use of adsorbents for NH₃ removal from emission sources or air is an urgent issue. However, there are difficulties in the compatibility between high adsorption capacity and recyclability for most conventional adsorbents. In this work, a structural transformation strategy using metal-organic frameworks (MOFs) is proposed for large-scale and recyclable NH₃ adsorption. A series of M(BDC) (M = Cu, Zn, Cd) materials can transform into one-dimensional M(BDC)(NH₃)₂ after NH₃ adsorption, resulting in repeatable adsorption capacities of 17.2, 14.1, and 7.4 mmol/g, respectively. These MOFs can be completely regenerated at 250 °C for 80 min with no adsorption capacity loss. Besides, breakthrough and cycle tests indicate that Cu(BDC) and Zn(BDC) show good performance in the removal of low concentrations of NH₃ from the air. Overall, combining the advantages of high adsorption capacity and recyclability due to the reversible structural transformation, Cu(BDC) and Zn(BDC) can be employed as ideal adsorbent candidates for NH₃ removal.